I wouldn't recommend migrating from one version to another. Depending on your code, multiple problems will arise.
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schaibaJan 29 '13 at 18:55

At a previous job, I had 2000, 2005, and 2008 r2 all on one machine without problems, altough some people had issues when initially installing multiple versions. It should work, based on a history of it working.
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thursdaysgeekJan 29 '13 at 19:14

I'd be very careful when installing SQL 2008 after SQL 2012 is already installed, especially if the 2012 is already a production instance. If you do go down that path, make sure you have solid backups of your server (and not just your databases) before you start. (If it were me, I'd find another machine/VM/something, and install 2008 there.) If you are installing SQL2008 first, then SQL2012, then forget I mentioned it.
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darin straitJan 29 '13 at 21:32